Floods in East Africa and India, as well as drought and fires in Australia, are periodic catastrophes caused by a second climate disruptor less well known than its cousin El Niño: the Indian Ocean Dipole. A new study that made the cover of Nature Geoscience, involving a researcher from the LOCEAN laboratory (IRD, UPMC, CNRS, MNHN) and various partners, shows that this recently developed phenomenon affects the climate in this part of the world. The researchers also show that the phenomenon has been occurring more frequently over the past 30 years. The number of extreme meteorological events that it causes should continue to increase in the coming years due to climate change.
A climate disruptor that is still misunderstood
Highlighted several years ago, the phenomenon called "the Indian Ocean Dipole" is a difference in temperatures between the surface waters in the West and East of this ocean. Like "El Niño" and its colder twin "La Niña" in the Pacific ocean, the Indian ocean dipole fluctuates every 3 to 8 years between "negative," "positive," and "neutral" phases. The new study shows that the oceanic anomalies induced, like those in the Pacific, periodically disrupt the climate in the Indian Ocean region by promoting extreme meteorological events.
Droughts in the East and Floods in the West
During the positive phases of the Indian Ocean Dipole, the western part of the ocean is colder than normal, while the eastern part is warmer. This surface water temperature anomaly changes atmospheric circulation. In the east, it reduces atmospheric convection (the rise of hot, humid air) and reduces precipitation. In the west, conversely, it increases convection. Moreover, this change in convection accelerates the trade winds along the equator, which causes colder deep water to rise and reinforces the temperature contrast between the two sides of the ocean. The positive phase of the Indian Ocean Dipole thus tends to cause droughts in East Asia and Australia, and, on the contrary, floods in some parts of the Indian subcontinent and East Africa.
Increasingly frequent events
Through the analysis of oceanic and climatic observations and to complex computer simulations on data going back to the middle of the 19th century, the researchers also showed that the positive phases have been increasingly frequent over the past 30 years. According to the study, this increased frequency is due to warming of the tropical zone of the Indian Ocean that occurs more rapidly in the west than in the east, in part due to the greenhouse effect or to the rising atmospheric temperature. The researchers identified a record number of eleven positive dipolar events since the 1980s. This frequency is predicted to increase further in the decades to come with the constant rise in surface temperature of Earth.
This work will help better predict the extreme climatic events that occur in this part of the world and to better anticipate their consequences. It will thus help governments and populations to better prevent collateral damage such as major forest fires in Southern Australia, the deterioration of the coral reefs west of the island of Sumatra, or even the increase in floods and malaria epidemics in East Africa.
- Wenju Cai, Xiao-Tong Zheng, Evan Weller, Mat Collins, Tim Cowan, Matthieu Lengaigne, Weidong Yu, Toshio Yamagata. Projected response of the Indian Ocean Dipole to greenhouse warming. Nature Geoscience, 2013; 6 (12): 999 DOI: 10.1038/NGEO2009
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